The present invention relates to a method for forming images by irradiation of electron rays and to a novel resist composition utilizable for the method. More particularly, the present invention relates to a method for forming images wherein a stereocomplex of poly(methyl methacrylate) is used as sensitivve material and subjected to an image-forming exposure treatment under irradiation of electron rays and thereafter the exposed material is developed with an organic solvent, and to a new resist composition utilizable for the method.
The use of poly(methyl methacrylate) is already known to form a resist capable of affording a positive image when subjected to irradiation of electron rays.
In case poly(methyl methacrylate) is used as a resist for forming images by irradiation of electron rays, a 5- 10 % solution of this polymer is first prepared and applied onto a substrate such as a silicon oxide panel to form a thin film of 0.3-0.5 .mu. in thickness. The thin film is then exposed to irradiation of electron rays through a desired original whereby poly(methyl methacrylate) in the exposed areas is decomposed to a low molecular compound. A desired positive image is formed when the exposed thin film is washed with a proper organic solvent as developer to wash off the low molecular compound thereby attaining development.
However, the poly(methyl methacrylate) which has been used previously is a mixture of methyl methacrylate polymers having different molecular weights containing low molecular polymers. On development of the exposed material, therefore, not only the low molecular weight compounds produced in exposed areas but also those contained in unexposed areas are washed off with a developing solvent, thus causing the formation of pinholes. This is one of the detrimental imperfections in the resist.
To overcome these drawbacks in the prior art method using poly(methyl methacrylate), a process has been proposed wherein poly(methyl methacrylate) is dissolved homogeneously in a good solvent and a poor solvent or a non-solvent for the polymer is added in small portions to the solution to effect separation of phases for removing low molecular weight components in the starting material. Another proposal is the use of a process wherein a combination of a good solvent and a poor solvent or non-solvent which have been properly mixed to a desired dissolving power is used on development to wash off only the low molecular weight compounds formed by decomposition of the polymer at the time of irradiation by electron rays.
In the use of such mixed solvent, however, the good solvent alone tends to separate from the other solvent and become selectively adsorbed to the poly(methyl methacrylate) layer, thus making it difficult to eliminate the low molecular weight components effectively. Therefore, these previous procedures have not been completely satisfactory due to the formation of pinholes, reduction in processing accuracy, reduction in resolving power of the resist and lack of clarity of image lines.
In order to overcome the above mentioned deficiencies, it has now been found in accordance with this invention that when syndiotactic poly(methyl methacrylate) obtained by a usual radical polymerization is mixed with poly(methyl methacrylate) obtained by anionic polymerization and containing a large proportion of polymer of isotactic structure, or with poly(methyl methacrylate) having isotactic and syndiotactic polymer blocks, a stereocomplex of the polymer is formed having extremely low solubility in organic solvents used in the developing treatment; and that when the stereocomplex is exposed to irradiation of electron rays, it is easily decomposed to form low molecular weight compounds soluble in the organic solvent.